High velocity accuracy

[rsterne]

You need to read some articles on ballistics, and how and why the wind causes a projectile to drift.... It is the lag time that is critical, and that increases dramatically in the transonic region (Mach 0.8-1.2) because of the huge increase in drag.... This is not just a theory, but has rock solid data from thousands of ballistics test to back it up.... It may be counter-intuitive to you, but those are the facts....

Bob

[K.O.]

hey Bob where did the Sonic choking thing end up..? some of the convo got past my concentration level...had my little theory... and only understood some arguments... but have stuff to look up to understand it all...

so...dropped out...

was consensus ever reached..?

[Tomygun1]

Thanks Bob,
And I am on the same page as you.    I understand the published studies on lag time that you refer to.    The topic is why aren’t we producing pellets that have a correct sectional density/weight/ and BC to travel at 1040fps at sea level better than we are now.    It may turn out that it requires a longer 20cal projectile with a very rapid rifling twist to get the best results, I don’t know.    With firearms 26cal is the magic number for maximum velocity/sectional density.    But I do know that all calibers in pellet guns can be improved on over what we now accept.    What I do know is that since I was 13yrs old pellet performance has only slightly improved.    I am an octogenarian and back then it was also thought that an airplane couldn’t exceed the sound ‘barrier’.    A lifetime in shooting sports hasn't left me uninformed and where we seem to differ is only with lag time.    It is my position that lag time is overcome with velocity and improved BC and that seems to be being ignored and has created a ‘barrier’ keeping air rifle velocities slow.   Clearly its not a problem in supersonic velocities but we are talking subsonic speeds, and like with high velocity there is a point of diminishing returns not to be exceeded.    It’s my position that we aren’t yet close to the ‘sweet spot’.


Thanks for your knowledge,
Tom

[K.O.]

"It may turn out that it requires a longer 20cal projectile with a very rapid rifling twist to get the best results, I don’t know."

No... any projectile with CG in front and high drag at the rear needs treated like a dart...less twist will always be better... 

[Tomygun1]

Twist will always be related to the length of the projectile.    At the velocity we are discussing I’m not convinced that a boat tail design is the answer but it couldn’t hurt.    CG in front and drag at the rear is the enemy.

[K.O.]

We have those they are slugs... we/ Bob Started doing them long ago... we also have little pointy slugs flat base and cup base... The boat tail Are called Bobs Boat Tail...

and yes and no to your enemy statement... there are benefits to pellets and in the last five years heavy pellets have become more prevalent... things have evolved...

[K.O.]

and a long dart will need less twist than slug...

even at subsonic a boat tail has advantages...

[K.O.]

yes .257 is a bit of a sweet spot for slugs with air power...

[Tomygun1]

Thanks Kirby,


I get the slug thing and shoot them well and pellets also.    But try to understand just how limited pellets in our modern pcp rifles are these days due to their horrible sectional density and BC.    Length and weight distribution will be a factor in improving these projectiles and no one will disagree that the longer the projectile is the faster it must spin to stabilize.    Not the same principle as with a skirted narrow wasted non aerodynamic blunt pellet.    Still not talking slugs, a longer slim hollow pellet spinning faster can only perform better at the same weight than the lead darts we call pellets that we now shoot.


I hope that you, like me, want to improve on what we have come to accept as the best it can be.    What we have works great at short range.


Tom

[rsterne]

Pellets require very little spin to be accurate, start thinking about 100 calibers per revolution, up to 200.... That means 22-44" twist rates for a .22 cal, and pellet length has little to do with it, as pellets are flare stabilized.... There is an excellent article on this in this thread.... https://www.gatewaytoairguns.org/GTA/index.php?topic=169459.0

Pellets behave in a similar way to darts, but do require a slow spin to even out imperfections in their construction.... Pellet weights, and hence their SD, have increased greatly in the last decade.... A heavy .22 cal 15 years ago was 21 gr., now they go all the way up to 34 gr., a 60% increase in SD.... I was one of the first to experiment with slower twist, I designed a 26" twist barrel in .30 cal which won the EBR the first year it was used.... Since then, FX in particular have led the way in slower twist barrels for pellets, although the slowest are still found in benchrest rifles like the Thomas BR, which can shoot 25.4 gr. pellets at 100 yards into dime-sized groups....

Bob

[ballisticboy]

Length and weight distribution will be a factor in improving these projectiles and no one will disagree that the longer the projectile is the faster it must spin to stabilize.    Not the same principle as with a skirted narrow wasted non aerodynamic blunt pellet.    Still not talking slugs, a longer slim hollow pellet spinning faster can only perform better at the same weight than the lead darts we call pellets that we now shoot.


I hope that you, like me, want to improve on what we have come to accept as the best it can be.    What we have works great at short range.


Tom

It is not as simple as a longer projectile needing a higher or lower spin, particularly something which uses aerogyro stabilization. The optimum barrel twist rate will depend on the size of the aerodynamic stabilizing moment and how much supplementary gyroscopic stabilization is needed to minimize group sizes. If the long projectile has much larger aerodynamic stabilization moments, then a lower twist rate may be needed, but this will also depend on the ratio of the axial and transverse moments of inertia. The barrel twist rate will also have to keep clear of the projectile's resonant frequencies, normally being kept at a higher twist rate above the resonant frequency rather than lower. Both the moments of inertia and the resonant frequencies will depend on the mass distribution of the projectile as well as the aerodynamics.
You may well also find that a longer projectile, which depends on flare stabilization, will have a higher drag and thus a lower BC due to the air seeing more of the flare at small yaw angles.
As for cross wind response, thinking in BCs and velocity is not really helpful, you really need to think in terms of aerodynamic drag coefficients and how they grow as speed approaches the speed of sound (1116.5 ft/sec). If the correct reference drag law is being used, then the BC should not change or change by very little as the speed increases, which tends to hide from people's thoughts the increase in the drag coefficient. Thus, thinking about BC and velocity alone will not give you a true picture. The fact is that, particularly for a blunt nosed projectile be it slug or pellet, the drag will increase. There comes a point where the drag increase more than offsets the use of a higher velocity, giving an increase in the lag time and thus the down wind drift.

[Cableaddict]

As for cross wind response, thinking in BCs and velocity is not really helpful, you really need to think in terms of aerodynamic drag coefficients and how they grow as speed approaches the speed of sound (1116.5 ft/sec). If the correct reference drag law is being used, then the BC should not change or change by very little as the speed increases, which tends to hide from people's thoughts the increase in the drag coefficient. Thus, thinking about BC and velocity alone will not give you a true picture. The fact is that, particularly for a blunt nosed projectile be it slug or pellet, the drag will increase. There comes a point where the drag increase more than offsets the use of a higher velocity, giving an increase in the lag time and thus the down wind drift.

Miles,

How does this relate to SD?

I'm thinking specifically of the difference between a 34 grain 22 cal slug, and a 34 grain 25 caliber slug, both with the same nose and tail shape. 

Obviously, the 22 cal will be longer / thinner.  Based on past discussions, it seems that the 22 cal will have better BC,  and thus might be a better choice for long-range hunting, if a flatter trajectory is preferred. 
-  but what about lag time / wind resistance?


(And of course, barrel design may also play a major part in this,  but I'm hoping that there's a reasonable theoretical answer when using a "typical" modern pcp rifle.  )

[K.O.]

...and higher drag of  the .25 is what creates it having more lag time... if that's what you are asking...

[rsterne]

If the Form Factor is similar, then the higher SD of the .22 cal compared to a .25 cal of the same weight will results in a better BC.... The equation for BC is....

BC = SD / FF

and a .22 cal will have a 29% higher SD than a .25 cal of the same weight....

Bob

[JungleShooter]

/
The equation for BC is....

BC = SD / FF

and a .22 cal will have a 29% higher SD than a .25 cal of the same weight....

I really didn't like math in school.
If my teachers would just have done math as Bob does it — I would have had much better grades — Bob makes it fun!

Lovin' the simplicity of a formula — that spells out complicated relationships of aerodynamics, physics, the works. Just plug in the numbers, and —— out comes the result. 👍🏼
Fascinating.

Matthias

[Baco]

what about in practical terms
.22 wins on BC which needs distance to take advantage of, wind drift goes to the .22
efficieny to the .25
is there an example how much % more energy would you get on .25 with the same gun same weight projectile?

[Cableaddict]

If the Form Factor is similar, then the higher SD of the .22 cal compared to a .25 cal of the same weight will results in a better BC.... The equation for BC is....

BC = SD / FF

and a .22 cal will have a 29% higher SD than a .25 cal of the same weight....

But what about barrel design?

If someone made a 177 cal rifle specifically designed in every way to optimally shoot a 50 grain 177 cal HP slug,  that would probably be a great hunting gun. (Assuming the slugs expanded enough upon impact.)

- But that rifle doesn't exist.   Nor does one optimally tuned for a 34 grain 22 cal,  and I don't expect to see one anytime soon.

And if I've understood most of the above, the 34 gr 22 cal & 25 cal projectiles will require different twist rates, for optimum "immunity" to wind.

So given the "typical" twist rates, chokes, and barrel lengths,  (yeah, I know....)   which of these slugs has the best chance of long-range accuracy in the field?

[rsterne]

Roughly speaking, with the same weight pellet in the same gun, but changing from .22 to .25 cal has the POTENTIAL to increase the power (FPE) by about 29%, but only if the porting is full bore size in both cases.... If you optimize the gun in .22 cal and then just change to a .25 cal, about half that increase (14-15 %) is typical.... It may use a bit more air to produce that, but generally the efficiency of the larger caliber (in FPE/CI) will be slightly greater....

The 25.4 gr. JSB Monster in .22 cal and 34 gr. Monster in .25 cal have reputations as shooting well.... The 34 gr. Beast in .22 cal has been challenging to get the best accuracy from, but I'm sure the right barrel will eventually be found.... Presently I don't know what that might be....

Bob

[Tomygun1]

Hypothetically speaking lets look at the possibilities inherent in a 177cal pcp rifle projectile as proposed by Allan.    A hollow bullet shaped non skirted ‘pellet’ around the weight he suggests would have a length of about .5” and a tremendous SD (sectional density) compared to the pellets we now shoot, and SD in a projectile when stabilized by a proper rifling twist can overcome a lot to increase the practical range of accuracy and retained energy.    The smaller diameter 177cal punches a smaller hole in the air with less frontal resistance, the longer projectile is subject to greater side affects from wind, while the greater SD and BC struggles to overcome that.    I can see a net gain here.


I am reminded of the studies done by the US military after adopting the .556cal over the previous.308 ammo.     They were so pleased with the results that it led to the development of the ‘needle gun’.    As a field weapon it was a failure but the principle was sound and it worked.    Stabilize a long thin projectile with a high SD.    The pressure used in our pcp rifles and the reasons we use them might be reason enough to take a second look at this technology to increase practical range and accuracy.    I’m not looking to resurrect the needle gun, but lessons learned apply to increasing range and retained energy with good SD numbers with pellet rifles.     


Hypothetically.

[ER00z]

Without digging too deep into "the science" of high velocity accuracy, I will say at shorter ranges accuracy can be had at 1000+ fps. This probably has been covered already, apologies for back stepping if so. But I've found 30 to 45 yards (if you're lucky) to be about the limits for pellets, slugs can go at least double that at 1000fps+. There are a few 177 pcp's capable of over 30fpe and slugs can be pushed fairly quickly in comparison to pellets. If you choose a regulated pcp, make sure you have a big plenum or potential (peak) power will suffer.

Another thing to consider, is lead fouling. Every airgun that I've pushed velocities into the 1000+fps range, the barrels lead up badly. I would think a slower twist brass barrel (or barrel liner) would be the way to go, but it's just a thought. 

Although not shot from a pcp (But a springer, mod 135), here's a recent group I've had at 25 yards pushing .177 10.5gr piranhas at 1005-1010fps. Four went into the cluster with one outlier. Haven't had the chance to push the distance further.